Proper calibration of tools, bone structures, implants and other components used in computer assisted surgery (CAS) procedures is vital.
In particular, determining the center of rotation (COR) of a spherically shaped object for use during a CAS surgery is a fairly common, but nonetheless important procedure. For example, during a total hip replacement (THR) surgery, determining the COR of the partially spherical femoral head and/or the corresponding cup-shaped acetabulum within which it is received, is typically required in order to ensure proper relative positioning of the respective femoral head and acetabular cup implants.
At least two known methods are currently employed for determining the COR of such a spherical object using a CAS system. For simplicity, these methods will be briefly described with reference to calculating the center of rotation of a femoral head. The first method involves rotating the femur between several positions, and capturing position and orientation information at each of the positions using the CAS system, from which the CAS system is able to determine the center point about which the femur is rotating by extrapolating lines from each of the captured positions and determining an intersection point thereof. More specifically, the femur is first maintained in a stable position such that the CAS system is able to register its position in space. The femur is then rotated to another position, and the position capturing procedure is repeated. This is repeated in order to permit the CAS system to identify and capture at least three distinct positions of the femur, from which the CAS system can define and calculate an imaginary cone having a tip coincident with the COR of the femoral head about which the femur was rotated between measured positions. Alternately, another method involves gradually rotating the femur in space during which time the CAS system automatically collects position and orientation information of the femur at predetermined regular intervals. These methods are simple, however have certain drawbacks. Particularly, if only three points are captured, the error margin remains relatively high. However, capturing a plurality of points, while improving accuracy, can be overly time consuming. Additionally, if the surgeon or user is not careful to displace the limb through its full rotational envelope and the points are captured too close to each other (i.e. linearly or quasi-linearly), then the resulting cone calculated by the CAS system will be skewed and not representative of the true COR of the limb. Further, another disadvantage of this method is the fact that it requires the surgeon to hold and rotate the limb of the patient through a relatively large region above the operating table, which in certain cases can at the very least be quite awkward. Other possibility for errors exists with these methods. For example, any displacement of the femoral head within the acetabulum as it is rotated therewithin, additionally adds error to the calculation of the tip of the cone and therefore the calculated center of rotation can differ from the true center of rotation of the limb by a significant amount.
A second method which as been employed to determine the COR of a spherical object using a CAS system involves using a tracked pointer or digitizer to collect a number of points on the spherical surfaces itself. Given a sufficient number of points on the surface, the CAS system is then able to reconstruct or digitize the surface, from which it can calculate an estimated center of rotation thereof. This method, however, requires relatively complex calculations on the part of the CAS system and further can result in imprecise results caused by an imperfectly digitized surface. This method also requires that a plurality of points on the surface of the spherical surfaces be digitized in order to provide accurate results.
Accordingly, there remains nonetheless a need for an improved device and method for determining the center of rotation of a spherical object using a CAS system.